Nucleotides form the structural unit (i.e., monomer) of nucleic acids. Each nucleotide comprises a sugar, a phosphate, and a base. In each of RNA and DNA, the sugar and phosphate molecules are the same in each nucleotide, with variation arising from the choice of four different bases.
Nucleotides can be divided into two classes based on the chemical structure of their bases. Purines constitute adenosine and guanine, whereas pyrimidines correspond to cytosine, thymine, and uracil. The only way that a DNA molecule can maintain a constant fixed width is if base pair interactions (including the wobble base pairing of guanine to uracil in RNA) occur only between purines and pyrimidines.